I can't speak for the appropriateness, sorry! But my advice is to wear layers. This time of year I usually do some sort of long sleeved shirt, then a sweater or down vest, depending, and then my rain jacket over top. It's usually warm enough, but you can pull layers off if it happens to get nice. As far as snow showers, I saw that on one forecast site, but not on the national weather service and the overall trend seems to not be really cold, so I think probably what they mean is that it won't snow hard, or only for a brief period of time and won't stick or will melt quickly. Overall, I don't thing you need to be concerned, if you are coming in on Friday, with any luck anything they get will be over and gone. But better to be a few hours late, or stay the night in Portland and drive down in the morning if there are any questions. The website
www.tripcheck.org is really good and will tell you if there are hazardous road conditions.